Apple Pay is a new mobile payment service for both online and offline retailers. You can use a brand new iPhone 6 or Apple Watch for in-store transactions. You should also use a phone to pay within apps. Checkout consists of 2 steps. First, you have to select Apple Pay. After that you should place a finger on a fingerprint scanner.
With Apple Pay you can store all your credit and debit cards in Passbook. You can import them from iTunes account, use iSight camera, or type a card information in manually. The first added card becomes your payment card by default, but you can change this in Settings.